If the power demand exceeds the power supplying capability, the circuit breaker is thrown in a limited power system such as in homes and buildings, and the power system will lose power. If an apparatus that requires continuous operation such as a medical apparatus is connected to the power system, the occurrence of a blackout is fatal. Moreover, in the case of an overall regional transmission grid, if the power demand exceeds the power supplying capability, for example, during severe heat in the summer, there is a possibility that a blackout will occur since the supply of power may fall short and cause considerable damage.
In order to avoid this kind of problem, known is a demand control technology of selecting one or more electric apparatuses among a plurality of electric apparatuses and controlling such electric apparatuses to be in a power-saving operation state in order to prevent the total power consumption of the plurality of electric apparatuses connected to a network from exceeding the upper limit value of the power supply. For example, known is a control device which targets air conditioners that consume large amounts of power and controls such air conditioners, in order from the lowest priority; for instance, from an air conditioner of an air-conditioned room with few people, to the power-saving operation state (refer to Patent Document 1). Also known is a demand control system which expanded the demand control target to home appliances other than air conditioners (refer to Patent Document 2).
In the demand control described in Patent Document 1 or Patent Document 2, the power-saving operation state of an apparatus of low priority is continued, and the user of such apparatus in the power-saving operation state is inconvenienced for a long time.    Patent Document 1: Japanese Patent Application Laid-open No. H10-30834    Patent Document 2: Japanese Patent Application Laid-open No. 2004-363661